1. Field of the Invention
The present invention relates to a conductive antireflection film stacked on a transparent substrate to reduce reflection of light by the surface of the transparent substrate.
2. Description of the Prior Art
In a CRT of a television receiver or the like, a conductive antireflection film is used to prevent an image from being obscured by reflection of external light or prevent an electric discharge from occurring between the CRT and a human body or the like owing to charging during an operation.
As such a conductive antireflection film, the present applicant proposed a four-layered conductive antireflection film in Japanese Patent Publication No. 4-15443. This film is formed by sequentially stacking an ITO layer, an MgF.sub.2 film, a TiO.sub.2 compound film and an MgF.sub.2 film on a transparent substrate in the order named.
The basic arrangement of the four-layered conductive antireflection film will be described below. Letting n.sub.i be the refractive index of the ith layer, d.sub.i is the geometric thickness of the ith layer and k.sub.i is the value obtained by dividing an optical film thickness (n.sub.i d.sub.i) by a 1/4 wavelength (.lambda..sub.0 / 4), the following equation can be established: EQU k.sub.i =4n.sub.i d.sub.i /.lambda..sub.0
where .lambda..sub.0 is the middle wavelength in design. If the value obtained by letting the value k.sub.i contain an incident angle .theta. as a parameter is represented by .delta..sub.i, then EQU .delta..sub.i =2.pi.n.sub.i d.sub.i cos.theta./.lambda..sub.0
In this case, as indicated by "Prior Art" in Table 1, the optimal basic arrangement of a film that satisfies required optical and electrical characteristics are designed such that the k values of the first to fourth layers are defined as follows: k.sub.1 =0.38, k.sub.2 =0.27, k.sub.3 =2.10 and k.sub.4 =1.00, provided that a first layer 31 as an ITO layer, a second layer 32 as an MgF.sub.2 film, a third layer 33 as a TiO.sub.2 compound film and a fourth layer 34 as an MgF.sub.2 film are stacked on a transparent substrate 10 in the order named, as shown in FIG. 1.
The preferable ranges of these k values are:
093.ltoreq.k.sub.4 .ltoreq.1.05 PA1 1.97.ltoreq.k.sub.3 .ltoreq.2.15 PA1 0.25.ltoreq.k.sub.2 .ltoreq.0.37 PA1 0.34.ltoreq.k.sub.1 .ltoreq.0.50
Note that the film thickness range of a given layer is determined as an allowable film thickness range based on the assumption that the thicknesses of the remaining layers are accurately controlled within the respective film thickness ranges defined in the basic arrangement of the film.
FIG. 2 shows the spectral reflectance of the conventional film shown in Table 1. Note that the film arrangements in Tables 1 and 2 are expressed by the k values. As is apparent from FIG. 2, in the wavelength range of about 430 nm to 680 nm, the reflectance is 0.3% or less, exhibiting excellent optical characteristics. In addition, the sheet resistance is 300 .OMEGA./.quadrature. or less, exhibiting excellent electrical characteristics.
Although the above-described conventional film has excellent optical and electrical characteristics, the film is scratched by a load of 300 gf in a scratch test using the tip of a mechanical pencil as a test indenter.